The invention concerns a method for assembling two flat or substantially flat parts, such as plates or sections, by means of fixations such as bolts, rivets etc.
More precisely, the invention relates to the optimised assembly of two parts designed to bear and transmit predetermined forces, either uniform or varying in time, which may differ from one assembly to another.
The method according to the invention applies to all assemblies of substantially flat parts, whether metallic or in composite material, which use fixations such as rivets or bolts. It finds particularly advantageous use in aeronautics, in which this type of assembly is widely used.
In an aircraft, bolted or riveted assembly is the most frequently used assembly mode. A passenger transport or cargo aircraft comprises more than one million rivets and close to 300000 bolts.
In assemblies of this type, fixations perform the functions of force transfer, sealing and the transmission of static electricity current and lightening.
The design of assemblies using bolts and rivets is therefore vital for the performance of the entire structure of the aircraft. Any poor design, would lead to a limited lifetime and weight excess.
In the current state of the art, the positioning of rivets and bolts is made according to the usual practice of each aircraft builder without having true recourse to any particular methodology.
The subject of the invention is precisely a method for assembling two substantially flat parts, metallic or in composite material, aimed at optimising the positioning of the fixations used to produce this assembly, so as to guarantee controlled, optimum lifetime.
In accordance with the invention, this result is obtained by means of a method for assembling two substantially flat parts, using at least one end line of fixations intended to transmit determined forces between said parts, and oriented in the plane of the parts, characterized in that it consists of calculating a ratio R between the tensile stresses "sgr"T and bending stresses "sgr"F relating to said forces, and of orienting said end line of fixations along an angle xcex1 relative to the direction of the neutral fibre of said parts, the absolute value of said angle xcex1 being such that: 10.8 ln (R)+16  less than |xcex1| less than 13.9 ln (R)+28 when the ratio R is at least 1, and such that 16 less than |xcex1| less than 28 when the ratio R is less than 1, the absolute value of angle xcex1 being no more than 90xc2x0.
The applicant has established by test-supported numeric simulations, that by positioning the first line of fixations along a direction xcex1 paying heed to the above-defined range of values, it is possible to better distribute the stresses borne by each of said fixations, and consequently to increase the lifetime of the assemblies in respect of fatigue and static resistance.
Under one preferred embodiment of the invention, the end line of fixations is oriented along an angle xcex1 substantially equal in absolute value to 11.6 ln (R)+21 when the ratio R is at least 1, and substantially equal to 20xc2x0 when the ratio R is less than 1.
If the forces to be transmitted are alternate forces, the end line of fixations is advantageously oriented along the above-mentioned angle xcex1 and along an angle xe2x88x92xcex1, either side of the neutral fibre of the parts. The fixation furthest in front is then on the neutral fibre.
The assembly method of the invention may be used both when parts are assembled directly one to the other and when they are assembled via one or two fishplates.
In the former case, the two parts are directly assembled one to another by at least two end lines of fixations oriented along angle xcex1.
In the latter case, that is to say when the two parts are assembled one to another via a fishplate, each of the parts is fixed to the fishplate by at least two end lines of fixations. Advantageously, these end lines are then oriented along angle xcex1. As a variant, at least the end lines the furthest away from the other part are oriented along angle xcex1.
In the third case, that is to say when the two parts are assembled one to the other via two fishplates, each of the parts is fixed to the two fishplates by at least two end lines of fixations, among which at least the end line the most distant from the other part is oriented along angle xcex1.
The invention applies both to parts having a substantially constant thickness in the assembly zone and to parts whose thickness tapers towards the ends in the above-cited zone.